1. Field of the Invention:
The present invention relates to a carburetor for internal combustion engines and, more particularly, a slow fuel supply system incorporated in the carburetor for supplying fuel particularly during a slow speed or idling operation.
2. Description of the Prior Art:
In usual carburetors for automobile internal combustion engines, when the throttle valve is brought near to a full closed position for operating the engine at a slow speed or is brought to an idling opening position which is nearer to the full closed position for effecting an idling operation of the engine, almost no vacuum is generated in the venturi portion thereof and, accordingly, no substantial amount of fuel is drawn from the main nozzle. Because of this, the carburetor is provided with a slow port which opens adjacent an end of the throttle valve located at the idling opening position and an idle port which opens at a little downstream side of the slow port, whereby it is effected that a small amount of fuel is drawn out toward the intake passage by a vacuum generated in the small air stream which flows through a small clearance left between the end of the throttle valve and a wall portion of the intake passage where the slow port opens or an intake vacuum applied to the idle port.
In actual carburetors, an air bleed is incorporated in a fuel supply passage provided therein for provisionally mixing a proper amount of air into the stream of a liquid fuel such as gasoline flowing through the fuel passage and such an air bleed is also incorporated in the slow fuel supply system which supplied fuel to the aforementioned slow port and idle port. With respect to the air bleed, it is said that it is more effective to supply a less amount of air at more individual portions located along a fuel supply passage.
In a conventional slow fuel supply system having two air bleeds, a slow jet element and an economizer jet element, a stream of fuel is generated by the vacuum applied to the slow port and the idling port to flow through a slow fuel passage starting from a float chamber, passing through a metering portion in the slow jet element where the stream is metered and then being ejected from a fuel ejecting port of the slow jet element into a joining region of the slow fuel passage and a first air bleed so that the fuel is first mixed with air supplied from the first air bleed thereby producing a fuel-air mixture. This fuel-air mixture is then metered again when it passes through the economiser jet element, and, thereafter, is mixed with air supplied from a second air bleed and then the fuel-air mixture flows toward the slow port and the idle port. In the structure in which a slow port plug is inserted into a part of the slow fuel passage connecting to the slow port, the passage flow area of said part is reduced by the plug thereby effecting an increase of the flow speed of the fuel-air mixture which accelerates the atomization of the mixture and provides a good mixing condition of the mixture delivering from the slow port and the idle port.
Although a conventional carburetor of the aforementioned structure operates satisfactorily for the primary object of effecting a slow or idling operation, actually a fluctuation of about 100 r.p.m. in the rotational speed of the engine is unavoidable and, therefore, if it is sought to lower the idling rotational speed in view of the fuel economy or the exhaust gas emission, the above-mentioned fluctuation in the rotational speed of the engine becomes a serious problem which bars ensuring a stable idling operation of the engine.
Therefore, in order to obtain a stable idling operation at a very low rotational speed, there now exists a requirement for an improvement of the slow fuel supply system so that the fluctuation in the engine rotational speed at a very low speed rotation is minimised or, in other words, the fuel supply from the slow fuel system is more stabilized.
In view of the above mentioned situation, we have made experimental researches about the behavior of the fuel flowing through the slow fuel supply systems by employing visualized models and a high speed camera, expecting to obtain a precise knowledge of the above mentioned behavior of fuel. As a result, we have found that in a low speed operation or when the intake air flow rate is slow, the supply of fuel from the fuel ejecting port of the slow jet element to the joining portion of the fuel passage and the air bleed passage and the supply of fuel at the slow port are both made in an intermittent manner.